1. Field of the Invention
The present invention relates to a color video camera, particularly to the method of generating luminance signals thereof.
2. Description of Related Art
FIG.1 shows an arrangement of color filters of an image sensor described, for example, on pages 76 through 82 of "National Technical Report", Vol. 31, No. 1, February 1985, composed by Matsushita Techno Research Co., Ltd. and published by Ohmsha Publishing Co., Ltd. In FIG. 1, Mg represents a pixel having a magenta color filter, G represents a pixel having a green color filter, Cy represents a pixel having a cyan color filter and Ye represents a pixel having a yellow color filter. FIG. 2 shows a part of a signal processing circuit of a color video camera which employs an image sensor consisting of these color filters arranged thereon. In FIG. 2, numeral 1 indicates a lens, numeral 2 indicates an image sensor, numeral 3 indicates a band-pass filter (BPF, numeral 4 indicates a detector, numeral 5 indicates a one horizontal period delay circuit (1HDLY), numeral 6 indicates a switching circuit and numeral 45 indicates a low-pass filter (LPF).
The operation will now be described below. In FIG. 2, incident ray on the lens 1 forms an image on the image sensor 2. In FIG. 1, an output signal from line n of the image sensor 2, consisting of a sequence of (Mg+Cy) and (G+Ye) being repeated, is denoted as Sn, and an output signal from line n+1 of the image sensor 2, consisting of a sequence of (Mg+Ye) and (G+Cy) being repeated, is denoted as Sn+1. Then Sn and Sn+1 are represented by the following equations. EQU Sn=Yn+Cn.multidot.sin(.omega.t)+ . . . (1) EQU Sn+1=Yn+1+Cn+1.multidot.sin(.omega.t)+ . . . (2)
Where .omega. is the carrier frequency of the color signal which corresponds to double the horizontal pixel width. Yn and Yn+1 in equations (1) and (2) represent the luminance signal components of line n and line n+1, Cn and Cn+1 represent the color difference signal components of line n and line n+1, respectively, and are given by the following equations. EQU Yn=(Ye+G)+(Cy+Mg)=2R+3G+2B (3) EQU Yn+1=(Ye+Mg)+(Cy+G)=2R+3G+2B (4) EQU Cn=(Cy+Mg)-(Ye+G)=2B-G (5) EQU Cn+1=(Ye+Mg)-(Cy+G)=2R-G (6)
The luminance signal components Yn, Yn+1 are obtained by passing the output of the image sensor 2 through the low-pass filter 45. The color difference signal components Cn, Cn+1 are obtained by passing the output of the image sensor 2 through the band-pass filter 3 having a center frequency .omega. and a detector 4. The output of the detector 4 gives 2R-G and 2B-G appearing every two lines. These signals 2R-G, 2B-G which appear on every other line are synchronized by the one horizontal period delay circuit 5 and the switching circuit 6.
In the conventional color video camera as described above, an output signal of the image sensor is passed through a low-pass filter to remove the modulated components of the color signal and obtain a luminance signal, resulting in a problem of the harmonics of the luminance signals being attenuated. Although aperture correction has been made by enhancing the rising edge and falling edge of the signal to improve the resolution, it causes an impression of unnatural enhancement.
FIG. 3 shows a block circuit diagram illustrating the circuit of a color video camera employing a spatial offset of 3-chip CCD color camera which is described, for example, on pages 1079 through 1085 of the "Journal of Television Engineering Association", November 1988. In FIG. 3, numeral 51 indicates a lens, numeral 52 indicates a refracting prism which decomposes incident ray into three colors of red, green and blue, numeral 53, 54, 55 indicate image sensors, numeral 56 indicates a red signal amplifier, numeral 57 indicates a green signal amplifier, numeral 58 indicates a blue signal amplifier, numeral 68 indicates a low-pass filter, numeral 71 indicates an adder and numeral 73 indicates a demultiplexer. FIG. 4 shows the constitution of output signals with the conventional method of spatial offset of 3-chip CCD color camera. In FIG. 4, G represents the signal of a green pixel, and RB represents the composite signal of red and blue pixels. Letter p indicates the horizontal pixel width of the image sensor. Green image sensors and red, blue image sensors are arranged in the horizontal direction at intervals of a half pixel width.
The operation will now be described below. In FIG. 3, incident ray on the lens 51 is decomposed into red, green and blue by the refracting prism 52, with the light rays of respective colors forming images on image sensors 53, 54, 55. Each of the image sensors 53, 54, 55 mixes signals of the upper and lower adjacent pixels to give one signal output. The output signals R, G, B of the image sensors 54, 55 are amplified by the red signal amplifier 56, the green signal amplifier 57 and the blue signal amplifier 58, respectively, so that the ratio of the output signals thereof becomes, in the case of NTSC system, R:G:B=0.30:0.59:0.11, to obtain R', G', B' signals. R' and B' are mixed in the adder 71 to obtain a signal RB which combines R' and B'. The demultiplexer 73 switches alternately between G' and RB to produce an output of luminance signal, which is passed through the low-pass filter 68 to obtain a luminance signal Y. Consequently, the luminance signal Y is given by equation (7). ##EQU1##
In the conventional method of spatial offset of 3-chip CCD color camera, the purpose is set at improving the resolution. Although there arises no problem in the case of such objects that have green signal and red-blue combined signal in similar proportions, but vertical lines appear in the case of objects which have significantly different proportions. Vertical lines have been reduced by passing the green signal and red-blue combined signal through a low-pass filter in the prior art, though it has a problem of causing attenuation of harmonics in the luminance signal. Thus resolution has been improved by enhancing the rising edge and falling edge of a signal for aperture correction, resulting in a problem of unnatural enhancement.